The Collaborative International Dictionary of English v.0.48:

Rhachis \Rha"chis\, n.; pl. E. Rhachises, L. Rhachides. [See
   Rachis.] [Written also rechis.]
   1. (Anat.) The spine.
      [1913 Webster]

   2. (Bot.)
      (a) The continued stem or midrib of a pinnately compound
          leaf, as in a rose leaf or a fern.
      (b) The principal axis in a raceme, spike, panicle, or
          corymb.
          [1913 Webster]

   3. (Zool.)
      (a) The shaft of a feather. The rhachis of the
          after-shaft, or plumule, is called the hyporhachis.
      (b) The central cord in the stem of a crinoid.
      (c) The median part of the radula of a mollusk.
      (d) A central cord of the ovary of nematodes.
          [1913 Webster]